1. Field of Invention
The present invention relates generally to motor speed control, and more particularly to a method of controlling rotational speed of a motor, wherein the motor's rotational speed is also regulated according to an ambient temperature surrounding the particular exothermic components currently being cooled by a cooling fan operated by the controlled motor. The present invention relates also to a control circuit for implementing the method.
2. Description of Related Arts
Various electronic apparatuses usually have exothermic components that generate heat when the electronic apparatus operates. To cool the exothermic components, fans are often used to evacuate warm air from enclosures in which electronic systems are contained. For example, in personal computers, servers, or the like, the operating frequencies of the CPUs become increasingly higher causing the heat values of the CPUs to increase. Accordingly, computers or similar electronic devices include one or more cooling fans to aid in circulating the air inside the enclosures and for maintaining the ambient temperature inside the enclosures within an acceptable range. The increased airflow provided by fans typically aids in eliminating waste heat that may otherwise build up and adversely affect system operation. Employing cooling fans is especially helpful in ensuring proper operation for certain central processing units (CPUs) with relatively high operating temperatures. These fans are operated by DC controlled motors. Similarly, in power supplies or the like, the loadings of the power supplies become increasingly higher causing the heat values of the exothermic components to increase. Accordingly, power supplies or similar electronic devices include one or more cooling fans to aid in circulating the air inside the enclosures and for maintaining the ambient temperature inside the enclosures within an acceptable range. The increased airflow provided by fans typically aids in eliminating waste heat that may otherwise build up and adversely affect system operation. Employing cooling fans is especially helpful in ensuring proper operation for components of the power supplies with relatively high operating temperatures. These fans are operated by DC controlled motors. In fact, the cooling fans operated by DC controlled motors can be used in many applications to cool all kinds of exothermic components which require cooling.
For the purpose of conserving electric energy and lowering noise caused by unnecessarily high airflow, the rotation speed of the motor is often expected to be controllable. For instance, the motor is generally required to operate at its full speed when the CPU overheats, while when the CPU is already cooled down or in a stand-by mode, the rotation speed of the motor is expected to be lowered accordingly in order to save energy and reduce annoying noise.
As disclosed in U.S. Pat. No. 7,425,812, control of fans in a system typically involves a fan control unit executing a fan control algorithm. A fan control algorithm may determine the method for controlling one or more fans that are configured to evacuate warm air from a system enclosure. For example, the fan control algorithm may specify that a fan's speed should be increased or decreased dependent upon a detected ambient temperature. Such control algorithms may also involve lowering the speed of the fan and allowing the fan to continue running at a minimum speed.
As disclosed in U.S. Pat. No. 5,687,079, it suggests controlling the speed of the fan in a computer according to the temperature of the ambient air detected by a thermistor. The current supplied to the motor operating the fan is controlled by a transistor. The base of the transistor is connected to a circuit comprising a thermistor. A low temperature voltage divider provides a constant low voltage to the motor when air temperature is below 28° C. A high temperature voltage divider provides a constant high voltage to the motor when air temperature is above 40° C. Thus, voltage supplied to the motor is constant below 28° C., increases linearly, and is again constant above 40° C. Referring to FIG. 1, as disclosed in U.S. Pat. No. 6,617,815, a perspective view of a curve of the voltage supplied to a motor according to the ambient temperature in the conventional circuit of the prior art is illustrated. As shown in FIG. 1, the vertical axis shows the voltage and the horizontal axis shows the ambient temperature measured at the temperature sensor. Below a lower temperature Tmin, a constant voltage Vmin is supplied to the motor. Above a higher temperature Tmax, a constant voltage Vmax is supplied to the motor. Between Tmin and Tmax, the voltage supplied to the motor is a linear function of the ambient temperature, and varies between Vmin and Vmax.
For a temperature control of the type shown in U.S. Pat. No. 5,687,079, three independent parameters need to be set. The first is the minimal fan speed Umin. The second is the lower temperature Tmin below which the fan will operate at its minimum speed. The third one is the upper temperature Tmax above which the fan will operate at its maximum speed. The prior art circuits do not allow easy control of these three parameters; however, these parameters need to be adapted to the type of computer system in which the fan is used.
Referring to FIG. 2, as disclosed in U.S. Pub. No. 2005/0047762, a perspective view of a curve of the rotation speed of a motor relative to an ambient temperature in the conventional circuit of the prior art is illustrated. As shown in FIG. 2, the vertical axis shows the rotation speed of the motor and the horizontal axis shows the ambient temperature measured at the temperature sensor. When the ambient temperature is lower than a lower temperature Tmin, the motor operates at a lower constant rotation speed Umin. When the ambient temperature is higher than a higher temperature Tmax, the motor operates at a higher constant rotation speed Umax or its full rotation speed. When the ambient temperature is in the range of Tmin to Tmax, the rotation speed of the motor is a linear function of the temperature, and varies between Umin and Umax.
It would thus be desirable to provide a motor speed control method and a control circuit for implementing the motor speed control method in order to conserve electric energy and lower noise caused by unnecessarily high airflow such that the power loss and the power efficiency could be improved.